Many types of vacuum arc coating apparatus utilize a cathodic arc source in which an electric arc is formed between an anode and a cathode plate in a vacuum chamber. The arc generates a cathode spot on a target surface of the cathode, which evaporates the cathode material into the chamber. The cathodic evaporate disperses as a plasma within the chamber, and upon contact with one or more substrates coats the substrates with the cathode material, which may be metal, ceramic etc. An example of such an arc coating apparatus is described in U.S. Pat. No. 3,793,179 issued Feb. 19, 1974 to Sablev, which is incorporated herein by reference.
An arc coating apparatus of this type is advantageous for use in the coating of large substrates and multiple substrates, due to the large surface area of the cathode which can be evaporated into a large volume coating chamber. However, in a large surface area cathode arc coating apparatus of this type a significant portion of the target evaporation surface of the cathode plate goes largely unused, due to the scanning pattern of arc spots which follows certain physical principles:
1. The arc discharge tends to move in a direction which reduces the voltage drop in the arc circuit, and the arc spot thus tends to migrate to regions on the target surface which are closest to the anodic current conductor. Where multiple current conductors traverse the cathode the arc spot will occasionally migrate into the region between conductors where it may remain for a considerable time because no steering mechanism is present to move the arc spot back to the desired evaporation zone.
2. In the case of metal cathodes the arc spot follows a retrograde motion according to the “anti-ampere force” principle, and is thus attracted to the coaxial magnetic force lines generated by the anodic current conductor.
3. In the case of a cathode formed from a material which does not have a melting phase, for example a sintered or graphite cathode, the arc spot moves according to the “ampere force” principle and is repelled from the coaxial magnetic force lines generated by the anodic current conductor.
4. The arc spot is attracted to the region where the tangential component of a transverse magnetic field is strongest.
5. The arc spot tends to migrate away from the apex of an acute angle at the point of intersection between a magnetic field line and the cathode target surface (the “acute angle” rule).
These effects result in a limited erosion zone relative to the available area of the target surface of the cathode plate, reducing the life of the cathode and dispersing cathodic evaporate into the coating chamber in non-uniform concentrations.
In a large area cathode arc coating apparatus using a metal cathode plate the anti-ampere motion of the arc spot and the tendency of the arc to seek the lowest voltage drop combine to largely confine the arc spot to the vicinity of the anodic conductor, substantially limiting the erosion zone to the region of the target surface surrounding the anodic conductor. This results in a very small area inside the coating chamber in which the cathodic evaporate is concentrated enough to apply a uniform coating to the substrates. However, it is not possible to construct the cathode plate so that the desired coating material is located only in the erosion zone, since the arc spot will occasionally stray out of the erosion zone and if the target surface is not entirely composed of the selected coating material the cathodic evaporate from outside the desired erosion zone will contaminate the coating on the substrates.
In the case of a cathode plate formed from a material which does not have a melting phase, the tendency of the arc spot to move in an ampere direction, away from the region of the anodic conductor, is opposed by the tendency of the arc discharge to settle toward the region of lowest voltage drop. In these cases the arc spot tends to move chaotically over the target surface of the cathode and the cathodic evaporate accordingly disperses in random locations and non-uniform concentrations within the coating chamber, rendering uniform coating of the substrates improbable. This random motion also causes the arc spot to move off of the target surface of the cathode and causes undesirable erosion of non-target portions of the cathode plate, for example the side edges.
U.S. Pat. No. 4,448,659 issued May 15, 1984 to Morrison, which is incorporated herein by reference, describes an arc coating apparatus providing a cathode in the form of a plate with a large target surface for creating cathodic evaporate. A confinement ring composed of a magnetically permeable material surrounds the cathode to confine the arc spot to the target surface. Such plasma sources can be used for the production of coatings on large and long articles, but present the following disadvantages:
1. Despite the initial low probability of the presence of cathodic spots on the protective ring, over time the cathodic evaporate coats the ring and cathodic spots are produced on the ring with increasing frequency. This results in contamination of the coating by the ring material, and ultimately in ring failure.
2. In self-steering cathodic arc sources it is not possible to use external magnetic fields in the vicinity of the target surface of the cathode. It is therefore not possible in such an apparatus to use a plasma-focusing magnetic field, as the influence of the focusing magnetic field makes the distribution of cathodic spots on the working surface of the cathode irregular and non-uniform. Any external magnetic field, for example for focusing or deflecting the arc plasma flow, interferes with the self-sustained magnetic field generated by the cathode and anode current conductors and disrupts the self-steering character of the cathode spot. However, the absence of magnetic focusing reduces the efficiency of the coating process and impairs the quality of substrate coatings, because the content of the neutral component (macroparticles, clusters and neutral atoms) in the region of the substrates, and thus in the substrate coating, increases.
3. A cathode in this type of plasma source rapidly becomes concave due to evaporative decomposition, and its useful life is therefore relatively short. Moreover, since the evaporation surface of the cathode becomes concave in a relatively short time it is practically impossible to use a high voltage pulse spark igniter in such a design, so that a mechanical igniter must be used which lowers working reliability and stability.
4. While the confinement ring prevents the arc spot from straying off of the target surface, it does not affect the tendency of the arc spot to migrate toward the anodic conductor in the case of metal cathodes, or to move chaotically over the target surface in the case of non-metal cathodes.
Accordingly, self-steering arc plasma sources tend to use the target surface inefficiently and the cathode thus has a relatively short useful life.
The erosion efficiency of the target surface can be improved by providing an arc spot steering system to steer the arc spot along a selected path about the target surface. This increases the size of the region within the coating chamber in which coating can occur.
For example, the scanning pattern of a cathode spot can be controlled by providing a closed-loop magnetic field source disposed beneath the target surface of the cathode, in a manner similar to that described in U.S. Pat. No. 4,724,058 issued Feb. 9, 1988 to Morrison, which is incorporated herein by reference. The magnetic field source establishes a magnetic field in a selected direction over the target surface, which directs the cathode spot in a direction substantially perpendicular to the direction of the magnetic field and thus provides more efficient evaporation of the target surface. This approach is based on the principle of arc spot motion whereby an arc spot is attracted to the region where the tangential component of a transverse magnetic field is strongest.
However, this still significantly limits the area of the target surface of the cathode which is available for erosion, because this type of arc coating apparatus creates a stagnation zone in the region where the tangential component of the magnetic field is strongest. The cathode spot eventually settles in the stagnation zone, tracing a retrograde path about the erosion zone and creating a narrow erosion corridor on the target surface. This limits the uniformity of the coating on the substrates and reduces the working life of the cathode.
U.S. Pat. No. 5,997,705 issued Dec. 7, 1999 to Welty, which is incorporated herein by reference, teaches a rectangular cathode plate in which the evaporation surface is located on the peripheral edge of the cathode plate, the arc spot being confined to the evaporation surface by cathode shields disposed over opposed faces of the cathode plate. Deflecting electrodes disposed about the cathode plate direct the plasma stream in two directions, parallel to the faces of the cathode plate.
In this apparatus the substrates must surround the edges of the cathode plate, and the cathode plate occupies most of the space within the apparatus. Thus, in order to coat a significant number of substrates the cathode plate, and thus the apparatus itself, must be extremely large. Also, filtration in this apparatus is poor, since the substrates are directly exposed to droplets and macroparticles entrained in the cathodic evaporate.
A deflecting electrode is described in U.S. Pat. No. 5,840,163 issued Nov. 24, 1998 to Welty, which is incorporated herein by reference. This patent teaches a rectangular vacuum arc plasma source in which a deflecting electrode is mounted inside the plasma duct and either electrically floating or biased positively with respect to the anode. However, this device requires a sensor which switches the polarity of the magnetic field when the arc spot on the rectangular source has reached the end of the cathode, in order to move the arc spot to the other side of the cathode. This results in an undesirable period where the magnetic field is zero; the arc is therefore not continuous, and is not controlled during this period. Consequently this ‘psuedo-random’ steering method cannot consistently produce reliable or reproducible coatings.
U.S. Pat. No. 4,673,477 to Ramalingam proposes that the magnetic field source can be moved to shift the magnetic field lines and increase the utilization efficiency of the target surface. However, the mechanical adaptations required for such a system make the apparatus too complicated and expensive to be practical.
Where an external magnetic field is present the arc spot follows the “acute angle” rule, according to which the arc spot tends to migrate away from the apex of an acute angle at the point of intersection between a magnetic field line and the cathode target surface. The basic principle is that a cathodic spot formed by a vacuum arc in a fairly strong magnetic field (in the order of 100 Gauss), the force lines of which cross the surface of the cathode at an acute angle, will move in a reverse (retrograde) direction perpendicular to the tangential component of the field and, concurrently, displace away from the apex of the angle (for example, see Cathodic Processes of Electric Arc by Kesaev I. G., Nauka, 1968). This results in the arc spot settling beneath the apex of the arch-shaped portion of the magnetic field which projects over the target surface.
U.S. Pat. No. 5,587,207 issued Dec. 24, 1996 to Gorokhovsky, which is incorporated herein by reference, teaches that cathode spot confinement under a closed loop-type linear anode can be enhanced by a conductor which encases the anode to form a closed loop magnetic coil, with the magnetic field lines oriented in the direction shown in FIGS. 29 and 30 therein. Simultaneous use of both the closed-loop magnetic steering coil behind the cathode and a closed-loop linear anode in front of the target evaporation surface (with or without an enclosed magnetic coil) results in a synergistic improvement of arc discharge stability and thus cathode spot motion. The anode can be configured in any desired pattern, the configuration thereof being limited only by the periphery of the target surface. The arc spot will scan the target surface under the influence of the transverse magnetic field (in an ampere direction in the case of cathodes of carbon and related sintered materials or in an anti-ampere direction in the case of metal cathodes), virtually unaffected by the current flowing through the anodic conductor.
A disadvantage of this approach is that the arc spot will occasionally migrate from the selected erosion zone to another part of the cathode target surface where the intensity of the transverse magnetic field is small and, there being no means available to return the arc spot to the desired erosion zone, will stagnate in the low magnetic field region. In the case of a carbon-based cathode plate, when the velocity of arc spot movement is low enough the arc spot can settle in any stagnation zone where the transverse magnetic field is close to zero, and will not return to the erosion zone.
U.S. Pat. No. 5,435,900 issued Jul. 25, 1995 to Gorokhovsky, which is incorporated herein by reference, discloses a deflecting magnetic system surrounding a parallelipedal plasma guide, in which the deflecting conductors force the plasma toward the substrate holder. However, this patent does not address the problem of magnetically steering an arc spot around a rectangular cathode plate in the presence of a deflecting magnetic field.